The present invention relates to an image display device and a method of making the same, and particularly to an image display device suitable for increasing the number of pixels and a method of making the same.
A prior art image display device will be explained hereunder with reference to FIGS. 6 and 7.
FIG. 7 illustrates a structure of a prior art poly-Si TFT (thin film transistor) liquid crystal display device. There are a pixel area 120, a signal line shift register 121, a signal line selection switch 122, a gate line shift register 124 and a gate line drive buffer 125 formed on a substrate 109.
A plurality of pixels each comprising a poly-Si TFT 128, a pixel electrode 50 connected thereto and a pixel capacitance 129 formed by the pixel electrode 50 are arranged in a matrix in the pixel area 120, and a gate and a drain of each poly-Si TFT 128 are connected to a gate line 127 and a signal line 126, respectively. For simplicity, only one pixel is shown in FIG. 7.
One end of the gate line 127 is connected to the gate line drive buffer 125 which in turn is scanned by the gate line shift register 124. One end of the signal line 126 is connected to the signal line selection switch 122 which in turn is scanned by the signal line shift register 121. The signal line selection switch 122 is supplied with signals via an analog signal input line 123.
The following explains the operation of the prior art liquid crystal device. The gate line shift register 124 selects the gate lines 127 sequentially via the gate line drive buffer 125. Poly-Si TFTs 128 in pixels in a row corresponding to a selected one of the gate lines 127 are turned on. During this ON period, the signal line shift register 121 scans the signal line selection switch 122 sequentially. The signal line selection switch 122 connects the signal lines 126 to the analog signal input line 123 sequentially when the signal line selection switch 122 is scanned, and consequently display signals inputted to the analog signal input line 123 are written into the respective pixel capacitances 129 sequentially via the signal lines 126 and the poly-Si TFTs 128.
FIG. 6 illustrates a cross-sectional view of a poly-Si TFT 128 disposed in each pixel and also a cross-sectional view of one of poly-Si TFTs constituting the gate line shift register 124 or the signal line shift register 121 disposed around the pixel area 120 on the substrate 109 on which the poly-Si TFTs 128 are formed.
Here, for the sake of simplicity, I shall assume an n-channel TFT. The poly-Si TFT 128 disposed in each pixel comprises a gate 101, a channel region 150 formed of a poly-Si thin film, an n+ source region 102, an nxe2x88x92 source region 103, an n+ drain region 105, and an nxe2x88x92 drain region 104. The poly-Si TFT constituting the gate line shift register 124 or the signal line shift register 121 comprises a gate 106, a channel region 151 formed of a poly-Si thin film, an nxe2x88x92 source region 107 and an n+ drain region 108. Both the above-explained two poly-Si TFTs are identical in structure and fabrication process except for dimensions. The only exception is such that the poly-Si TFT 128 is provided with the nxe2x88x92 source region 103 and the nxe2x88x92 drain region 104 to reduce leakage current through the poly-Si TFT 128 serving as a pixel switch when it is off. In FIG. 6, reference numeral 109 denotes a quartz glass substrate, 110 is a gate insulating film, and 111 is a protective film.
The prior art as explained above is disclosed in detail in SID (Society for Information Display International Symposium) 94 Digest of Technical Papers, pp.87-90 (1994), for example.
As the number of pixels in the display device is increased, the operating speeds of peripheral circuits such as the gate line shift register 124 and the signal line shift register 121, are required to increase further.
But, with the prior art technique, there is a problem in that it is difficult to optimize the designs of the peripheral circuits and the pixel area independently of each other. If the gate insulating film is made thinner for the purpose of speed-up of the peripheral circuits, for example, the poly-Si TFT 128 in the pixel area becomes incapable of withstanding a high voltage required for driving the liquid crystal.
To solve the above problems, an image display device according to an embodiment of the present invention comprises a plurality of pixels arranged in a matrix, the plurality of pixels each being provided with a pixel switching means, a pixel electrode coupled with the pixel switching means for applying an electric field to a material producing an electro-optical effect; a common electrode for cooperating with the pixel electrode in driving the material producing an electro-optical effect; a plurality of gate lines each extending in parallel with each other and connected to a corresponding row of the pixel switching means for selecting the corresponding row of the pixel switching means in a predetermined order; a gate line driver circuit for driving the plurality of gate lines; a plurality of signal lines extending in such a manner as to intersect the plurality of gate lines for supplying display signal voltages to the pixel electrodes via selected ones of the pixel switching means; and a display signal voltage applying circuit for applying the display signal voltages to respective ones of the plurality of signal lines, wherein the pixel switching means each is comprised of a poly-Si TFT of a first type structure, the display signal voltage applying circuit contains a plurality of poly-Si TFTs of a second type structure, a gate of each of the poly-Si TFTs of the first type structure is disposed to oppose a first side of a first poly-Si thin film for forming a channel of the poly-Si TFT of the first type structure with a first gate insulating film interposed therebetween and is connected to a corresponding one of the plurality of gate lines, a gate of each of the plurality of poly-Si TFTs of the second type structure is disposed to oppose a first side of a second poly-Si thin film for forming a channel of each of the plurality of poly-Si TFTs of the second type structure with a second gate insulating film interposed therebetween, sources and drains of the poly-Si TFTs of the first and second type structures are disposed approximately in a same plane on a same substrate of the image display device, and the first side of the first poly-Si thin film and the first side of the second poly-Si thin film are on opposite sides of the same plane from each other.
To solve the above problems, an image display device according to another embodiment of the present invention comprises a plurality of pixels arranged in a matrix, the plurality of pixels each being provided with a pixel switching means, a pixel electrode coupled with the pixel switching means for applying an electric field to a material producing an electro-optical effect; a common electrode for cooperating with the pixel electrode in driving the material producing an electro-optical effect; a plurality of gate lines each extending in parallel with each other and connected to a corresponding row of the pixel switching means for selecting the corresponding row of the pixel switching means in a predetermined order; a gate line driver circuit for driving the plurality of gate lines; a plurality of signal lines extending in such a manner as to intersect the plurality of gate lines for supplying display signal voltages to the pixel electrodes via selected ones of the pixel switching means; and a display signal voltage applying circuit for applying the display signal voltages to respective ones of the plurality of signal lines, wherein the pixel switching means each is comprised of a poly-Si TFT of a first type structure, the display signal voltage applying circuit contains a plurality of poly-Si TFTs of a second type structure, a gate of each of the poly-Si TFTs of the first type structure is disposed to oppose a first side of a first poly-Si thin film for forming a channel of the poly-Si TFT of the first type structure with a first gate insulating film interposed therebetween and is connected to a corresponding one of the plurality of gate lines, a gate of each of the plurality of poly-Si TFTs of the second type structure is disposed to oppose a first side of a second poly-Si thin film for forming a channel of each of the plurality of poly-Si TFTs of the second type structure with a second gate insulating film interposed therebetween, sources and drains of the poly-Si TFTs of the first and second type structures are disposed approximately in a same plane on a same substrate of the image display device, the first side of the first poly-Si thin film and the first side of the second poly-Si thin film are on opposite sides of the same plane from each other, and the second gate insulating film is thinner than the first gate insulating film.
To solve the above problems, an image display device according to still another embodiment of the present invention comprises a plurality of pixels arranged in a matrix, the plurality of pixels each being provided with a pixel switching means, a pixel electrode coupled with the pixel switching means for applying an electric field to a material producing an electro-optical effect; a common electrode for cooperating with the pixel electrode in driving the material producing an electro-optical effect; a plurality of gate lines each extending in parallel with each other and connected to a corresponding row of the pixel switching means for selecting the corresponding row of the pixel switching means in a predetermined order; a gate line driver circuit for driving the plurality of gate lines; a plurality of signal lines extending in such a manner as to intersect the plurality of gate lines for supplying display signal voltages to the pixel electrodes via selected ones of the pixel switching means; and a display signal voltage applying circuit for applying the display signal voltages to respective ones of the plurality of signal lines, wherein the pixel switching means each is comprised of a poly-Si TFT of a first type structure, the display signal voltage applying circuit contains a plurality of poly-Si TFTs of a second type structure, a first gate of each of the poly-Si TFTs of the first type structure is disposed to oppose a first poly-Si thin film for forming a channel of the poly-Si TFT of the first type structure with a first gate insulating film interposed therebetween on a surface of a substrate of the image display device and is connected to a corresponding one of the plurality of gate lines, a second gate of each of the plurality of poly-Si TFTs of the second type structure is disposed to oppose a second poly-Si thin film for forming a channel of each of the plurality of poly-Si TFTs of the second type structure with a second gate insulating film interposed therebetween on the surface, an order of arrangement of the first gate and the first poly-Si thin film is reversed from an order of arrangement of the second gate and second poly-Si thin film, and the second gate insulating film is thinner than the first gate insulating film.
To solve the above problems, a method of making an image display device according to still another embodiment of the present invention comprises a plurality of pixels arranged in a matrix, the plurality of pixels each being provided with a pixel switching means, a pixel electrode coupled with the pixel switching means for applying an electric field to a material producing an electro-optical effect; a common electrode for cooperating with the pixel electrode in driving the material producing an electro-optical effect; a plurality of gate lines each extending in parallel with each other and connected to a corresponding row of the pixel switching means for selecting the corresponding row of the pixel switching means in a predetermined order; a gate line driver circuit for driving the plurality of gate lines; a plurality of signal lines extending in such a manner as to intersect the plurality of gatelines for supplying display signal voltages to the pixel electrodes via selected ones of the pixel switching means; and a display signal voltage applying circuit for applying the display signal voltages to respective ones of the plurality of signal lines, wherein the pixel switching means each is comprised of a poly-Si TFT of a first type structure, the display signal voltage applying circuit contains a plurality of poly-Si TFTs of a second type structure, a first gate of each of the poly-Si TFTs of the first type structure is disposed to oppose a first poly-Si thin film for forming a channel of the poly-Si TFT of the first type structure with a first gate insulating film interposed therebetween on a surface of a substrate of the image display device and is connected to a corresponding one of the plurality of gate lines, a second gate of each of the plurality of poly-Si TFTs of the second type structure is disposed to oppose a second poly-Si thin film for forming a channel of each of the plurality of poly-Si TFTs of the second type structure with a second gate insulating film interposed therebetween on the surface of the substrate, an order of arrangement of the first gate and the first poly-Si thin film is reversed from an order of arrangement of the second gate and second poly-Si thin film; the method including the steps of forming a thin film made mainly, of amorphous Si on the surface of the substrate after one of two steps of (a) forming the first gate and the gate insulating film and (b) forming the second gate and the second gate insulating film, and converting the amorphous Si into polycrystalline silicon.